1. Field of the Invention
This invention relates to a supercharger for use with an internal combustion engine. More particularly, the invention pertains to a supercharger having a variable effective area through which engine exhaust gas can enter the turbine. More particularly still, the present invention concerns a means for control of the flow rate through the turbine and the exhaust system back pressure against which the engine must work.
2. Description of the Prior Art
A supercharger is a device used with an internal combustion engine for increasing the output power of the engine by increasing the amount and pressure of the air inducted into the engine. The power of the engine is directly limited by the amount of air that can be inducted during the intake stroke. Therefore, by compressing the inlet air to a pressure higher than the ambient conditions a greater quantity of air is inducted and the output power of the engine increased. In spark ignition engines the maximum boost in pressure of the inlet air should be in the range between six and eight psi. because of the tendency of such engines to knock if higher pressure inlet air is admitted to the engine.
Increasing the pressure of the inlet air has conventionally been accomplished by a supercharger wherein hot engine exhaust gas is passed through a turbine that is fixed to one end of a driveshaft. A centrifugal compressor mounted at the opposite end of the shaft compresses ambient air to the preferred range of pressure boost. Rotation of the compressor pumps the air radially outward into a volute that carries the compressed air to the inlet manifold of the engine.
It is important that a supercharger produces the necessary range of air pressure boost over the full operating range of the engine. If the supercharger were designed to produce full boost at the lower range of engine operating speed, for example, near 1500 rpm. and the full volume of exhaust gas were used to drive the turbine, the inlet air would be increased to approximately 20 psi at the high speed range of operation, for example, near 6,000 rpm. A boost of inlet air pressure to this level may be harmful to the engine structurally and detrimental to its function. An excessive back pressure in the engine exhaust gas system develops as speed increases because the cross-sectional area of the turbine is too small to accommodate the higher flow rates of engine exhaust gas although the area may be sufficient for the engine exhaust gas flow rate corresponding to the 1,500 rpm. range. Whereas the inlet air may receive a boost of approximately 12 inches of Hg at 6,000 rpm; back pressure is approximately 26 inches of Hg. It has been estimated that 1 inch of Hg back pressure reduces the output horsepower by approximately one percent.
Rather than sizing the supercharger at the low range of engine speed, the supercharger may be sized to accommodate the flow rate of engine exhaust gas corresponding to the intermediate range of engine speed. If, for example, the supercharger is designed to produce full boost at 3,000 rpm., an excessive boost would still result at 6,000 rpm. To avoid this difficulty, conventionally, a large portion of engine exhaust gas is allowed to bypass the turbine, the energy from the exhaust gas being thereby wasted. At 6,000 rpm., fully one-half of the available engine exhaust gas would be wasted to avoid excessive boost. Even with this technique, however, a pressure drop through the turbine of approximately six psi is imposed on the engine exhaust gas system against which the engine must work. The back pressure in the exhaust gas system is a power loss, yet half the energy of the exhaust gas that could be used to produce the requisite boost is not used for this purpose.
Another technique commonly used in superchargers to avoid overboost and excessive back pressure provides a low flow rate of exhaust gas to the turbine at the lower range of engine speed and increased flow rate at higher speeds. This is accomplished by turbine nozzles whose flow area is made smaller at low engine speed and made larger as engine speed increases. Such adjustable turbine nozzles require complex control and are inherently expensive.